Vertical ionization potential benchmark for unitary coupled-cluster and algebraic-diagrammatic construction methods

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate calculation of vertical ionization potentials (IPs) and electron detachment energies is crucial in quantum chemistry.
• Unitary coupled-cluster (UCC) theory and algebraic-diagrammatic construction (ADC) schemes are widely used for these calculations.
• Evaluating the performance of different methods against high-level benchmarks is essential for method selection.

Purpose of the Study:

• To assess the accuracy of various UCC and ADC-based methods for calculating vertical ionization potentials and electron detachment energies.
• To compare these methods against a high-level benchmark: equation-of-motion coupled-cluster theory with single, double, and triple excitations (IP-EOM-CCSDT).
• To determine the necessity of computationally demanding methods like the Dyson expansion method (DEM) for static self-energy calculations.

Main Methods:

• Statistical evaluation of approximately 200 electron-detached states across 41 molecules.
• Comparison of second-order methods (IP-ADC(2), IP-UCC2) against IP-EOM-CCSDT.
• Assessment of strict third-order methods (IP-ADC(3), IP-UCC3).
• Investigation of static self-energy corrections using the Dyson expansion method (DEM) and an improved fourth-order scheme (Σ(4+)).

Main Results:

• Second-order methods (IP-ADC(2), IP-UCC2) exhibit modest accuracy with mean absolute errors (MAEs) around 0.6 eV.
• Third-order methods show significant improvement: IP-ADC(3) (MAE = 0.35 eV) and IP-UCC3 (MAE = 0.29 eV).
• The IP-UCC3 method demonstrates the best performance among the tested methods, closely followed by IP-ADC(3).
• Employing DEM or the simpler Σ(4+) scheme for static self-energy does not substantially improve IP-ADC(3) results, suggesting DEM is not necessary.

Conclusions:

• Third-order IP-ADC and IP-UCC methods provide accurate and reliable calculations for ionization potentials and electron detachment energies.
• The IP-UCC3 method is recommended for its high accuracy and efficiency.
• Computationally simpler methods for static self-energy are sufficient, negating the need for the more complex DEM.